U.S. Pat. No. 3,054,014, Gemsa, assigned to the assignee of the present application and the disclosure of which is hereby incorporated by reference, describes an electrode for high-pressure discharge vessels with gas or vapor filling, in which the electrode is made of a thorium dioxide containing tungsten rod. Lamps of this type are usually used with apparatus requiring a pointlike radiation source, for example for use in an optical or scientific system. Problems arise in connection with electrodes of this type due to instability of the arc between the electrodes. Due to such instability, intensity of radiation will vary. Further, the tip of the electrode which functions as a cathode may deteriorate. None of these problems arise in electrodes used in electrode tubes.
Requirements for arc stability, uniformity of light output and lifetime have increased recently. The use of such lamps has, additionally, increased; new fields of application have opened up with improvements in such lamps.
To understand the improvement by the present invention, a review of prior art is in order. Specific reference is made to British Patent 929,668, Jones, and European Patent 0 087 826, Gartner. Both of these disclosures generally are directed to electrodes for electron tubes. These electron tube electrodes use a material which is highly temperature resistant, that is, has a high melting point. The typical material is tungsten. The tungsten is doped with an electron emissive material, usually thorium oxide (ThO.sub.2). The proportion of thorium oxide may vary within a wide range, depending on the purpose of the electrode, for example between 0.1 to 5%, by weight. Due to the high operating temperature of the lamp, elementary emitter material is formed which, preferably by diffusion along the grain boundaries, travels to the surface. This process is determinative for the quality of the electrode and can be influenced in various ways. Additionally, doping, for example with potassium or aluminum, can change the grain structure so that the grain boundary diffusion is additionally facilitated.
It is also known to dope a metallic body with carbon, in order to facilitate reduction of the emitter material. For example, carbon reduces ThO.sub.2 to Th, and CO.sub.2 is generated. For this purpose an external carbide layer can be applied to the metallic body. The high diffusion rate of carbon ensures penetration into the metallic body, see for example the cited literature by Gessinger and Buxbaum, "Materials for Scientific Research", Vol. 10 (1975), page 295 et seq.